Phosphorus limitation of primary productivity in the eastern Mediterranean Sea

Although NO3- is generally considered to limit primary productivity in most of the world’s oceans, previous studies have suggested the Mediterranean Sea may be an exception. In this study of the southeastern Mediterranean, we found that all the PO43- was removed from the upper water column during the winter phytoplankton bloom in the core and boundary of a warm-core eddy, while measurable (0.3-0.6 µM) NO3- remained. The N:P (NO3-: PO43-) ratio in the core and boundary of the Cyprus eddy was 27.4 and the slope of the linear portion of the N vs. P scattergram with 25.5 with a positive intercept of 0.5 µM on the NO3- axis. A similar N:P ratio (28-29), slope (21-23), and intercept (0.9-1.1) was found for the water column across much of the southern Levantine basin. These data, taken together with the results of incubation experiments, lead us to conclude that the southeastern Mediterranean is strongly P limited. The degree of P limitation increases from west to east across the entire basin. We suggest that removal of PO43 by adsorbtion on Fe- rich dust particles may be an important process controlling the concentration of P in the water column.